1. Field of the Invention
The present invention relates to a precipitation hardenable martensitic stainless steel and a steam turbine blade using the precipitation hardenable martensitic stainless steel.
2. Description of Related Art
A low pressure stage blade used for a steam turbine is demanded to be lengthened so as to enhance the efficiency of a power generation of a steam power generation plant using the blade. Further, high strength of the steam turbine blade is required from a viewpoint of securing the safety of the blade because the centrifugal force applied to the blade is increased as the length of the blade is increased.
A steam turbine blade using a 12 Cr (chromium) steel is known in a prior art (see, for example, Japanese Laid-Open Patent Publication No. 2000-161006). The 12 Cr steel having a high strength allows the steam turbine blade to have a high safety profile.
Meanwhile, in a steam turbine plant to be constructed abroad, of which export is expected to be expanded in future, it is assumed that the water quality used for the plant abroad is poorer than that used in Japan. Accordingly, the strength of the steam turbine blade used in the plant abroad is required to be highly increased and the corrosion resistance thereof is also required to be improved corresponding to the water quality used for the plant abroad. When considering such requirements, the above mentioned 12 Cr steel is not suitable for a material of the steam turbine blade used in the plant abroad because the corrosion resistance of the 12 Cr steel is insufficient.
In a prior art, a precipitation hardenable martensitic stainless steel is known as a material of the steam turbine blade (see, for example, Japanese Laid-Open Patent Publication Nos. 2005-194626, 2005-232575, and 2008-127613).
In general, in a precipitation hardenable martensitic stainless steel, a relatively large amount of Cr is added and an addition amount of C (carbon) having a harmful influence against the corrosion resistance is restricted. This allows the precipitation hardenable martensitic stainless steel to have an excellent corrosion resistance.
However, precipitation hardenable martensitic stainless steels disclosed in Japanese Laid-Open Patent Publication Nos. 2005-194626 and 2005-232575 have a high Cr equivalent calculated as a ferritic element, and are likely to form s-ferrite. Further, the precipitation hardenable martensitic stainless steel forming δ-ferrite has a lowered mechanical property such as tensile strength or toughness. Moreover, such a precipitation hardenable martensitic stainless steel has a high Ni equivalent calculated as an austenitic element, and is likely to form residual austenite. Accordingly, the precipitation hardenable martensitic stainless steels disclosed in Japanese Laid-Open Patent Publication Nos. 2005-194626 and 2005-232575 have a disadvantage that the stability of the martensite is insufficient.
Further, a precipitation hardenable martensitic stainless steel disclosed in Japanese Laid-Open Patent Publication No. 2008-127613 comprises multiple types of precipitates which contribute to the precipitation hardening in the martensite structure. However, although multiple types of precipitates are included in the precipitation hardenable martensitic stainless steel, the amounts of the precipitates are small, resulting in lacking the sufficient strength or toughness thereof.